ARAM Destroys Your MMR
Let me start off by saying that your normal game mmr is tied between summoners' rift and ARAM. This is a tragedy for anyone who likes to play both games on the same account.
You see, if you spend enough time playing ARAM, your mmr will inflate severely, especially if you play enough to learn the tricks and methods of winning in that game mode. When you decide to play on summoners' rift, you will be forced to play against people up to 2-3 divisions above you (skill-wise). If you find this hard to believe, here you go:
http://i.imgur.com/Mdqm7kW.png[/img]
This issue pretty much encourages the use or ARAM-only accounts, which only skew the ARAM experience. I currently have two accounts in which I am dissuaded from playing normal summoners' rift games because my mmr is severely inflated.
The fix here is pretty simple; separate summoners' rift mmr and ARAM mmr. I did want to expose this issue but unfortunately I doubt anything will be done.